The irony in "The Destructors" is that the teenaged boys in the story destroy what is left of the beauty of the world around them rather than using their energy and talent to preserve and improve it. In destroying the lovely, architecturally significant home in their neighborhood that has survived the Blitz, they further impoverish their own lives.
It is clear that the home's owner, Old Misery, is trying to reach out to boys and build community. For example, he gives them three packets of chocolate. Rationing still went on in 1950s Britain, and chocolate was not necessarily easy to come by, but Old Misery makes the gesture. He is willing to share with the boys what he has.
But the boys are suspicious of his gift:
The gang were puzzled and perturbed by this action and tried to explain it away. “Bet someone dropped them and he picked ’em up,” somebody suggested.
Rather than see it as a kind gesture, the boys take it as a way to "bribe" them into not bouncing a ball against the side of his house. They bounce the ball to show they can't be bribed.
Ironically, if the boys had forged a relationship with the past that Old Misery represents, they all could have helped each other forge a path of creation; however, for them the world is entirely a material place, devoid of the spiritual, and they reject the beauty and graciousness that Old Misery's house represents. Greene implies that this lack of respect for the past is as destructive to England as the Blitz once was.